Original subtitles

Our planet is the greatest living puzzle in the universe.

A collection of worlds within worlds,

each one a self-contained ecosystem bursting with life.

But how do they work?

The intricate web of relationships and the influence of natural forces

makes each microworld complex and unique.

So, to discover their secrets, we need to explore them one by one...

..untangle their interlocking pieces,

and ultimately reveal the vital piece - the key to life itself -

hidden deep within each of nature's microworlds.

Over 99% of space available for life on Earth is in the sea.

From top to bottom, the ocean contains a volume of water

totalling 1.3 billion cubic kilometres.

It is the single largest ecosystem on our planet -

far, far larger than any terrestrial ecosystem.

It's also the least explored.

We know more about the surface of Mars than

we do about the majority of the marine environment.

This microworld is Earth's inner space.

Almost 90% of the ocean lies below a kilometre,

and over 75% is deeper than three kilometres.

The very deepest part of the ocean lies at 11 kilometres

and would easily engulf Mount Everest.

More people have walked on the moon than have visited

the deepest part of the ocean.

Below the upper sunlit waters is a foreboding world of darkness

and crushing pressures.

Despite this, it's home to countless living things.

Some of the most bizarre and unlikely creatures exist in the depths.

Even though they make up the majority of creatures

upon our planet, we know very little about them.

The vast majority of these float or swim in the water column,

a world without walls or sunlight.

But even at the very bottom of the ocean,

there is life somehow making a living on a flat abyssal plain

that spans the seabed between continents.

Recent discoveries in the deep sea have astounded the scientific

community and given us an insight into another world.

So just how do creatures exist in the depths?

What is the key that connects all these animals and allows them

to survive in a cold, dark, hostile world?

Our journey starts at the surface of the open ocean

in the North Atlantic.

As the sun sets, a large mass of animals appear as if from nowhere

to feed at the sea's surface...

..millions upon millions of them.

A bizarre array of jellyfish trail stinging tentacles,

or ingest food directly into their bodies.

Among the throng are animals that defy classification.

A small fish takes cover inside the body of this pelagic jelly

for a very good reason.

Large shoals of squid arrive to hunt at the surface.

By coming at night, they can avoid the eyes of daytime predators.

But not all animals need eyes to hunt.

DOLPHINS SQUEAK

Spotted dolphins use sonar in the darkness...

..targeting their prey with pinpoint accuracy.

So where do all these night-time animals come from?

They ascend from a world with little or no sunlight,

hundreds of metres down.

Every night, across the world's oceans, 100 million tonnes of these

animals rise from the depths.

It's by far the largest migration of animal life on the planet.

They come to exploit the abundance of food in the surface waters.

An abundance that exists

because of something that happens at a microscopic level -

photosynthesis.

Tiny algae and plants known as phytoplankton

use light from the sun to turn soluble carbon into organic matter.

This is known as primary production.

The by-product of photosynthesis is oxygen.

The waters up here are rich in this important element,

which is essential for life.

50 billion tonnes of phytoplankton is produced

in the upper oceans every year.

During spring in the North Atlantic,

when conditions are right, their blooms can be seen from space.

It forms the basis of the marine food chain

and is fundamental to all life in the ocean.

Phytoplankton is fed on by tiny animals known as zooplankton.

The most common are copepods.

These crustaceans are little more than a millimetre long

but they are the most numerous animal in the ocean.

In the North Atlantic, a cubic metre of seawater

may contain in excess of 100,000 of them.

Using their legs, they create currents which push

the microscopic phytoplankton into their mouths.

It's the sheer numbers of phytoplankton

and copepods that draw deep-sea animals up at night.

When dawn returns, the migration is reversed and the massive

army of deep-sea creatures sink back down into the darkness.

If we're to follow them, we must use a deep-sea submersible

capable of withstanding immense pressures.

Without one, we could not survive in their world.

As we descend, the sun's rays are absorbed and scattered.

At 200 metres, we leave the photic zone

and enter the first layer of the deep sea -

the twilight zone.

At this depth, there's less than 1% of the sunlight at the surface.

The pressure has increased twentyfold, and the temperature has

dropped to four degrees, but we find a world of extraordinary beauty.

With nowhere to hide in the twilight zone, the best disguise

is transparency.

Like this squid with a delicate glass-like body.

Almost nine centimetres long, this amphipod is a giant of its kind.

It's completely transparent, apart from its two enormous eyes

and central nervous system.

Another peculiar crustacean lives like a hermit within

the stolen body of a jellyfish.

This shell also houses her offspring.

Her habit of pushing this protective shell around has led to

the nickname of pram bug.

The longest jellyfish of all are the giant siphonophores.

Their tentacles, lined with rows of stinging cells,

can reach 40 metres long.

Of the countless billions of animals living below the photic zone,

only a fraction migrate into shallower water to feed at night.

So what do these animals feed on?

Looking out of the window of our submersible, we can see

a constant rain of particles slowly drifting down around us.

Known as marine snow, this is a vital food source for everything

living below 200 metres.

It rains down from the sunlit waters above.

But what exactly is it?

The density and exuberance of life at the surface of the ocean

far outweighs that of the deep sea.

This is where marine snow originates.

A pod of common dolphins prepare to hunt.

They're homing in on a shoal of mackerel swimming near the surface.

Waiting are flocks of shearwaters, hoping for the fish

to be driven within range.

The mackerel swim in a tight ball for safety.

Working as a group, the dolphins drive the bait-ball upwards

towards the waiting shearwaters.

Shearwaters dive deep to grab their share.

Caught between the birds and the dolphins,

the mackerel have nowhere to go, and the frenzy builds.

The commotion attracts a school of yellowfin tuna.

These two-metre-long fish are capable of bursts of speed

of 75 kilometres per hour.

This doesn't deter the shearwaters, which continue to feast

on the shoal beneath.

Eventually, nothing remains of the huge shoal

of mackerel, except for scraps of flesh and scales.

Leftovers, faeces, dead and dying plants and animals

have only one way to go, and that's down.

This is marine snow.

Millions of tiny creatures, such as this sea spider, filter the snow.

Its feathery appendages gathering particles that are then drawn

through its jaws.

As a result, the nutritional value of marine snow declines with depth.

Feeding activity also uses up

precious reserves of dissolved oxygen.

Without photosynthesis to replace it, oxygen decreases.

So, as we descend, life begins to thin out.

But it becomes ever more extraordinary.

At 500 metres, it appears completely dark to human eyes.

The pressure is 50 times what it was at the surface

and there's only the tiniest remnant of sunlight filtering through.

Animals here have adapted to cope with extreme pressure

and their eyes have evolved to become disproportionately large.

The owner of THIS pair gazes ever upwards into the gloom,

seeking the silhouette of its prey against the faint down-welling light.

To escape the attention of super-sensitive eyes, fish here have

evolved light-transmitting cells on the underside of their body.

Using graphics, we can see how these exactly match the background light.

With this, their silhouette breaks up, making them

appear almost invisible from below.

By 800 metres, the pressure is 80 times what it was at the surface

and the temperature is below three degrees centigrade.

Oxygen levels have also decreased to less than 5%

of what they were at the surface.

This is called the dead zone.

But something still manages to live here.

Vampyroteuthis infernalis - the vampire squid from hell.

Vampire squid have lived in the depths for 200 million years.

They share physical characteristics with octopus and squid

and are thought to be a missing link.

Highly specialised blood cells allow them

to live in this low-oxygen environment.

Despite its hellish name and fierce look,

vampire squid are placid creatures, averaging only

28 centimetres in length, and are completely harmless.

Get too close to a vampire squid

and it puts on the most amazing light display.

Bioluminescent bacteria in its arms and on its body

dazzle and confuse potential predators.

And with that, it disappears into the darkness.

Beyond 1,000 metres, we enter the dark zone, where not even

the faintest remnant of sunlight can penetrate.

Yet when we switch off the submersible lights,

we see bioluminescence.

There's life even here.

Furthermore, oxygen levels have risen and that's because we are now

in a deep-water current called the Great Ocean Conveyer.

When surface water, enriched in oxygen by photosynthesis,

meets polar ice, it sinks beyond the first 1,000 metres

and flows towards the equator...

..carrying oxygen to the very deepest corners of the abyss.

Here in the dark zone, we find the real monsters of the deep.

Oxygen may be plentiful here but food is scarce,

and bioluminescence is the only light available.

Most of the flashing lights we can see come from deep-water copepods.

They provide food for other deep-sea animals.

To a hunting squid, this flashing light looks like food.

But in the darkness, nothing is what it seems.

Fooled by bioluminescent bacteria living in the antennae

of an angler fish.

The angler fish can easily accommodate

the squid in its extendable stomach.

Many deep-sea fish have disproportionately large stomachs.

In this sparse, cold world, it might be many days between meals.

Natural selection has produced lures of all shapes and sizes,

each used to tempt prey to within easy reach.

This Wolftrap angler has a lure hanging amidst its formidable teeth.

In this dazzling battlefield, prey have developed

some surprising methods of escape.

Deep-water shrimps confuse attackers by spinning

and releasing a bioluminescent glue.

While the shrimp makes its escape, the glue sticks,

leaving the attacker glowing in the face of its own enemies.

The shrimp's red colour is also its camouflage.

In the dark zone, the eyes of most predators

are tuned to the blue or green of bioluminescent light.

So, to them, a red shrimp is almost invisible,

but not to one deep-water resident -

the dragonfish.

This predator has evolved red bioluminescent headlights

below its eyes that are sensitive to red light.

Its target doesn't see it until it's too late.

Descending ever further, we eventually reach the sea floor,

six kilometres down.

The pressure here is more than 600 times that of the surface,

and temperatures are close to zero degrees.

It takes many weeks for marine snow to descend this far.

Here, it forms a vast blanket of soft sediment over a kilometre thick -

the abyssal plain.

These plains cover a third of the Earth's surface.

By the time the snow gets here, it only contains

a fraction of its original energy.

Under such extreme conditions, we would expect the abyssal plain

to be lifeless.

But even here, we find life.

Deepwater sea urchins must sift large quantities of sediment

to survive.

The strange balloon-like sacs on their backs may be filled

with a noxious substance to deter predators.

Abyssal shrimps use their elongated antennae to feel for tiny

particles of food floating around in the darkness.

Life here may be sparse, but because the abyssal plains cover

such a large area of the Earth's surface,

they are home to some of the most numerous animals on the planet.

Fish have been found living as far down as eight kilometres.

This rattail is one of the most common species.

The white spots around its eyes are pores,

sensitive to the slightest movement -

vital in the inky blackness of the abyssal plain.

Just occasionally, in this nutrient-poor environment,

a feast arrives.

A dead tuna doesn't lie unnoticed for long.

This deep-sea conger eel has picked up the scent from far off.

Down here, a good sense of smell is a lifesaver.

The carcass also attracts the attention of one of the deep's

most elusive creatures -

the six-gilled shark.

These rarely seen sharks are active hunters

and can grow to five metres in length.

This type of shark has been around for at least 200 million years.

There are many opportunistic scavengers on the seafloor.

They'll detect even the tiniest scent in the water

and will move in from miles around.

Deep-sea crabs and scavenging arrowtooth eels

are soon joined by giant isopods.

Related to woodlice, these strange-looking monsters

are half a metre long.

Within hours, the carcass is stripped to nothing.

Even the bones are eaten.

Where food is at a premium, nothing is wasted.

When life was first discovered thriving in the abyss, it was

a complete surprise, but the depths revealed another secret

that shook the foundations of scientific thinking.

A secret that lay here on the mid-ocean ridges.

These undersea mountain ranges are the longest geological

structure on Earth, part of a continuous 65,000 kilometre chain

extending across the face of the planet.

This is the frontline of plate tectonics.

Here, the seafloor is parting at about two centimetres a year.

When submersibles first visited the ridges in the 1970s,

they found towering basalt chimneys known as black smokers

spewing hot water and hydrogen sulphide

from deep inside the Earth's crust.

At 400 degrees centigrade, these smokers are hot enough to melt lead.

It was here that they made

one of the most important scientific discoveries of the 20th century -

dense populations of marine animals

living in a toxic deep-sea environment

without any reliance on energy from the sun.

In one location, they found swarms of deep-sea shrimps.

In another, they found bizarre polychaete worms tolerating

water temperatures of 80 degrees centigrade.

No other animal on Earth is known to exist at such temperatures.

Clams, crabs and even fish live in large numbers around the vents.

How do all these animals survive in such densities,

in total darkness, under conditions

of scalding heat and intense pressure?

The answer lies in mats of bacteria that coat the chimneys.

These bacteria are primary producers.

They substitute for phytoplankton in a world without sunlight.

Drawing chemical energy from the vent,

they convert soluble carbon into organic matter -

a process known as chemosynthesis.

It is this bacteria, not marine snow, that is

the basis of the hydrothermal vent food chain.

But in order to exploit this resource,

they've had to adapt to living with high levels of hydrogen sulphide -

a poison as potent as cyanide.

These giant tube worms approach two metres long

and they incorporate vent bacteria within their bodies.

The bacteria provide the worms directly with food and energy.

Despite the toxic environment,

there's thousands of times more life at hydrothermal vents

than at the abyssal plain.

The sheer scale and diversity of creatures here has opened up

a whole new branch of research.

New species are being discovered on every single dive.

So, here, far away from the sun's rays, is a self-sufficient

community thriving in the most hostile of environments.

This may be a glimpse of how life exists on other planets.

Our journey into the deep-sea microworld has revealed

some of the most unusual life forms on Earth.

What we've found at every level is a diverse community of animals

adopting unique ways of surviving.

Many of these creatures live well beyond the sun's rays.

But there's one thing that ties them all together wherever they dwell -

the need for oxygen.

Even the bacteria that form the basis of hydrothermal communities

need oxygen to make organic matter.

And oxygen can only be produced in one way -

as a by-product of photosynthesis,

a process driven by the sun.

Even though the majority of ocean creatures remain oblivious

to its presence, ultimately it is the sun

that allows life to exist in the depths of the abyss.

The sun connects all life, from the surface...

..to the deepest reaches of the sea.

We still know more about our neighbouring planets than

we do the deep sea.

This inner space remains the last frontier on Earth -

a frontier that continues to evoke awe and wonder.

Who knows what discoveries are yet to be made in the largest

but least explored of all our microworlds?